The present invention relates generally to transition or refractory metallic thin-films, and more particularly to a low-stress and low-resistivity transition or refractory metal thin-film that can be used in a large format flat-panel display.
Thin-film adhesion onto substrates is a challenging problem. After deposition onto substrates, many types of thin-films do not stick firmly onto the substrate. They fail the tape test: Stick a scotch tape onto the substrate with thin-films, and remove the tape; thin-films with poor adhesion leave with the tape.
One reason causing the adhesion problem is that the films are stressed. Their natural state is to be curved with respect to the flat substrate. Thus, they want to be lifted off from the substrate. Typically, 1.5 to 2.0 E10 dynes/cm2 tensile stress for a 2000 .ANG. film is a stressed film. If the tensile stress is below 1 E10 dynes/cm2, that film is considered relatively low-stressed.
The stress problems are intensified in diverse film stacks. For example, in a flat panel display, there can be thousands of row and column buses. In one embodiment, the row electrodes are made of chromium films, which are covered by a resistive film and a dielectric film; on top of the dielectric film, there are the chromium column buses. The column buses have to conform to the profile of other underlying films, and there is a tendency for the chromium buses to break due to stress, especially at the dielectric/resistor interface. As the size of the display increases, the yield goes down. In fact, the yield of a prior art large format display with a 320 mm by 340 mm panel has been reduced to 0% due to stress-breakage of the buses.
One way to reduce stress in a thin-film is to dope the thin-film with nitrogen, such as introducing nitrogen into a chromium film. For example, a 5% nitrogen-argongas mixture in an in-line sputtering system reduces the stress in a chromium film from 1.5 E10 to 4.0 E9 dynes/cm2. However, such benefits are at the expense of an increase in the film's sheet resistance--from 2 to 8 ohms/sq., for example.
Many applications using thin-films require the films to have low sheet resistance. This is because a low resistive film consumes less power, generates less heat and reduces signal delay due to RC time constant.
It should be obvious from the foregoing that there is a dilemma. Nitrogen doping has been known to reduce stress; however, nitrogen significantly increases the film's sheet resistance, which is detrimental to many applications. Thus, there is a need to have thin-films with two mutually exclusive characteristics--low in stress and low in sheet resistance.